Soil Mechanics / Phase relationships and soil states

Consistency limits and soil state

Atterberg limits, plasticity and the liquidity index that places a clay on its state scale.

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Why this matters

For fine soils, how much water is present changes the soil from a liquid, to a mouldable plastic solid, to a brittle solid. The Atterberg limits mark those transitions, and comparing a soil's natural water content to them tells you how it will behave — soft and squeezable, or firm.

Three water contents mark the changes in a fine soil's consistency:

the liquid limit $w_L$ (or LL) — above it the soil flows like a liquid;
the plastic limit $w_P$ (or PL) — below it the soil is too dry to mould and crumbles;
the shrinkage limit — below it further drying no longer reduces the volume.

Between the plastic and liquid limits the soil is plastic — it can be remoulded without cracking. The width of that range is the plasticity index.

Plasticity and liquidity indices

\text{PI} = w_L - w_P, \qquad \text{LI} = \dfrac{w - w_P}{\text{PI}}

What it calculates: the plastic range, and where the natural water content sits within it

$\text{PI}$: plasticity index (%)
$\text{LI}$: liquidity index (dimensionless)
w: natural water content (%)

In plain terms: LI ≈ 0 means the soil is near its plastic limit (firm); LI ≈ 1 means it is near its liquid limit (soft, close to flowing); LI > 1 warns of a very soft, potentially sensitive soil.

Check yourself

A clay has PL = 22%, LL = 45% and a natural water content w = 40%. Roughly where does it sit?

Summary

Atterberg limits (liquid, plastic, shrinkage) mark a fine soil's consistency transitions.
Plasticity index PI = wL − wP is the width of the plastic range.
Liquidity index LI = (w − wP)/PI places the natural water content on that scale: ~0 firm, ~1 soft.